Aggie, it depends... you mentioned a Leonardo I think, so I guess you're going to use film? If you're using RC paper, it's EI is about 4. You need to know your pinhole diameter and focal length to work out an aperture. Use another lightmeter and extropolate (spelling?) to your aperture and film speed (if using paper). Easiest way of measuring your pinhole is to use your enlarger and measure the hole projected and divide by the magnification factor.